Feel like a failure in relationships

I am going to be 49 in January. I have a full time job, have a masters and take care of my elderly parents aged 82 and 85, in house with them, other siblings too busy to help. I had a son who is 28 going on 29 this year.

I have dated and unsuccessfully I haven't been lucky so I remain single. I feel sad and alone at times, I would love to build a life with someone....I don't know what I am doing wrong.

Does anyone have any advice to give?

Thanks